In 3rd Generation Partnership Project (3GPP), standardization of LTE-Advanced (LTE-A) has been advanced as a fourth generation mobile communication system for achieving, an additional high speed and high-capacity communication from Long Term Evolution (LTE) which is a developed standard of third generation mobile communication system. LTE-A has addressed an important problem for a cell end user to improve in a throughput, in addition to achieving of such a high speed and high-capacity communication, and relay technology for relaying a radio transmission between a radio base station apparatus and a mobile terminal device has been studied as a means for solving such a problem. It is expected that it can also enlarge coverage efficiently by using the relay, even at a location where securing of a wire backhaul link is difficult.
In the relay technology, there are a layer 1 relay, a layer 2 relay, and a layer 3 relay. The layer 1 relay is relay technology called a booster or a repeater, and is Amplifier and Forward (AF) type relay technology for performing power amplification of a downlink received RF signal from a radio base station apparatus and transmitting the amplified downlink received RF signal to a mobile terminal device. An uplink received RF signal from a mobile terminal device is also subjected to the power amplification similarly to be transmitted to a radio base station. The Layer 2 relay is Decode and Forward (DF) type relay technology for coding and modulating the downlink received RF signal from the radio base station apparatus again after demodulating and decoding, and transmitting the coded and modulated downlink received RF signal to the mobile terminal device. The layer 3 relay is relay technology for reproducing an user data after decoding the downlink received RF signal from the radio base station apparatus, in addition to the demodulating process and the decoding process, and then executing processes for transmitting the user data over the radio again (e.g., a secrecy process, a dividing/coupling process of the user data, etc.), and transmitting the coded/modulated user data to the mobile terminal device. Nowadays, in 3GPP, standardization with regard to the layer 3 relay technology has been advanced from viewpoints of improving in receiving characteristics due to noise elimination, reviewing of standard specification, and simplicity of implementation.
FIG. 1 is a diagram showing a brief overview of radio relay technology using the layer 3 relay. The radio relay station apparatus (Relay Node (RN)) of layer 3 relay is characterized to have a specific cell ID (Physical Cell ID (PCI)) different from that of the radio base station apparatus (evolved Node B (eNB)), in addition to executing a reproducing process, a modulation and demodulation process, and a coding and decoding processing of user data. Accordingly, the mobile terminal device (User Equipment (UE)) recognizes a cell B provided by the radio relay station apparatus as a cell different from a cell A provided by the radio base station apparatus. Moreover, since a control signal of physical layers (e.g., Channel Quality Indicator (CQI) and Hybrid Automatic Repeat reQuest (HARQ)) is terminated at the radio relay station apparatus, the radio relay station apparatus is recognized as a radio base station apparatus, as seen from the mobile terminal device. Accordingly, even the mobile terminal device having only a function of LTE can be connected to the radio relay station apparatus.
Moreover, it is assumed that a backhaul link (Un) between the radio base station apparatus and the radio relay station apparatus and an access link (Uu) between the radio relay station apparatus and the mobile terminal device is operated on a different frequency or the same frequency. In the case of the latter, if the radio relay station apparatus executes a transmission process and a reception process simultaneously, a transmitted signal is wrapped around a receiver of the radio relay station apparatus, unless sufficient isolation is securable in a transmitting and receiving circuit, and thereby causing interference. Accordingly, in the case of operation on the same frequency (f1) as shown in FIG. 2, it is necessary to perform Time Division Multiplexing (TDM) of radio resources of the backhaul link and the access link (eNB transmission and relay transmission), and control so that the transmission process and the reception process are not simultaneously executed in the radio relay station apparatus (Refer to Non Patent Literature 1.). Accordingly, the radio relay station apparatus cannot transmit a downlink signal to the mobile terminal device, during reception of a downlink signal from the radio base station apparatus, in a downlink, for example.